Electric machines for use in motor vehicles, in the form of the starting motor (motor) and the alternator (generator), are well-known. In modern vehicles, alternating current generators, which have a claw-pole configuration and are (separately) excited electrically, may be used as generators. As a rule, rectifiers based on semiconductor diodes may be used for rectifying the alternating current generated.
However, due to the increasing need for electrical energy in the vehicle, the effort to reduce fuel consumption and lower emissions, as well as the desire to be able to combine the advantages of an electric motor with those of an internal combustion engine, electric machines having a dual function, so-called starter generators, are also being used increasingly.
Starter generators are electric machines, which may be operated, in a vehicle, as electric motors or as generators, as a function of need. As generators, starter generators must be able to assume all of the tasks that the alternator conventionally has, namely, electrically powering the vehicle electrical system and charging the vehicle battery. As electric motors, when starting the internal combustion engine, starter generators must bring its crankshaft up to the required starting speed in a short time.
However, the use of the starter generators is not limited to the above-mentioned functions. In the case of a suitably high nominal output, in motive operation, a starter generator may assist the internal combustion engine with propulsion, e.g., while accelerating in the so-called boost operation, and for turbo-lag compensation. During braking, a portion of the braking energy may be recovered (recuperated) by a regenerative operation of a starter generator. Corresponding drive units are referred to as hybrid drives.
A re(start) of the internal combustion engine, which is as rapid as possible, is particularly desirable in vehicles having an automatic start-stop mechanism. To this end, it is necessary to build up, as rapidly as possible, the exciting current in the exciting coil (rotor) of the electric machine (conventional generator or starter generator) used in each instance.
As a rule, in corresponding electric machines, the exciting current may be set using a pulse-width modulated voltage signal, where in the case of continuous activation (thus, a pulse duty factor or pulse duty factor of the voltage signal of 100% or 1), an exciting current having a nominal current intensity flows. In this connection, it is a current intensity, at which an exciting coil of the machine is continuously driven in normal operation, and for which it is configured. After the pulse duty factor of 100% is set, the nominal current intensity is only reached with a certain delay, e.g., only after 300 ms, due to the high inductance of the rotor winding. This delay accordingly slows down the starting of the engine and is therefore not satisfactory.
Thus, there is a need for suitable improvements, in particular, in vehicles having an automatic start-stop mechanism.